The present disclosure relates to a composite joint of two adherents, either of the same or dissimilar materials, and a method for joining these adherents and a monopod device used in a cryoradiator that includes a metal and composite joint described herein.
Mechanical systems with very challenging performance goals require select materials (e.g., composite fiber reinforced laminates) with unique properties such as high strength, high stiffness, low thermal expansion, low thermal conductivity, etc. These select materials are utilized as key components in larger structural systems and therefore must be joined with adjacent members (typically made of dissimilar materials) to form composite joints. For example, such composite joints may include fiber reinforced laminates joined with adjacent metallic members. These composite joints typically use adhesives to bond the adjacent members or adherents together to properly transfer load.
Composite joints are necessary, and yet typically are the weakest link in the mechanical system. This is due to the fact that these composite joints are highly stressed. The high stress in the composite joint may be due to mismatches in thermal expansion properties and/or stiffness properties of the two adherents, extremely low temperature excursions (e.g., in cryogenic environments), thin composite members carrying high loads, and/or stress concentrations in transitional regions.
A typical composite joint which joins two members of different thicknesses and/or stiffnesses is shown in FIG. 1. It includes thicker or stiffer member M bonded to thinner or softer member C using adhesive A. FIG. 1. Also shows the load path L of an external load applied to member M. The external load applied to member M is transferred to member C via adhesive A.
The cryoradiator is a radiator that is configured to operate at very low temperatures and includes multiple thermal stages. Each stage is separated by a monopod that includes composite-to-metal joints. The monopod is an important component at the heart of the cryoradiator. The monopod includes thin-walled fiber-reinforced composite tubes bonded to metallic end fittings. The monopod functions both as a mechanical load path and a thermal isolator between the thermal stages of the radiator, which is operating at extremely cold temperatures. As a thermal isolator, the monopod provides a long path between the thermal stages of the radiator. The monopod is often designed to be compact enough to package it in tight confines.
The typical mismatched joint, where two adherents of different thicknesses/stiffnesses are bonded together, is not optimized for strength since it has (a) high stress concentrations at the leading edges of the adherents (where failure initiates) under mechanical loads and thermal-elastic loads and (b) high peel stresses (a typical failure mode for composite joints). The mismatched joint also provides many manufacturing problems (e.g., adhesive squeeze-out, voids, uncontrolled fillets, repeatability). When large members make up one or both of the adherents, the joint is very difficult to bond and align. Also, mechanical load-testing (i.e., at the part level) and non-destructive inspections are difficult to perform on the large members. Also, these mismatched joints make it difficult to develop consistent uniform bond line thicknesses which are critical to the joint's strength.
The present disclosure provides improvements over the prior art composite joints and/or methods for preparing such joints.